Java – ArrayList class

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Java ArrayList class hierarchy

Java ArrayList class uses a dynamic array for storing the elements. It inherits AbstractList class and implements List interface.

The important points about Java ArrayList class are:

  • Java ArrayList class can contain duplicate elements.
  • Java ArrayList class maintains insertion order.
  • Java ArrayList class is non synchronized.
  • Java ArrayList allows random access because array works at the index basis.
  • In Java ArrayList class, manipulation is slow because a lot of shifting needs to be occurred if any element is removed from the array list.

Hierarchy of ArrayList class

As shown in above diagram, Java ArrayList class extends AbstractList class which implements List interface. The List interface extends Collection and Iterable interfaces in hierarchical order.

ArrayList class declaration

Let’s see the declaration for java.util.ArrayList class.

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable

Constructors of Java ArrayList

Constructor Description
ArrayList() It is used to build an empty array list.
ArrayList(Collection c) It is used to build an array list that is initialized with the elements of the collection c.
ArrayList(int capacity) It is used to build an array list that has the specified initial capacity.

Methods of Java ArrayList

Method Description
void add(int index, Object element) It is used to insert the specified element at the specified position index in a list.
boolean addAll(Collection c) It is used to append all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection’s iterator.
void clear() It is used to remove all of the elements from this list.
int lastIndexOf(Object o) It is used to return the index in this list of the last occurrence of the specified element, or -1 if the list does not contain this element.
Object[] toArray() It is used to return an array containing all of the elements in this list in the correct order.
Object[] toArray(Object[] a) It is used to return an array containing all of the elements in this list in the correct order.
boolean add(Object o) It is used to append the specified element to the end of a list.
boolean addAll(int index, Collection c) It is used to insert all of the elements in the specified collection into this list, starting at the specified position.
Object clone() It is used to return a shallow copy of an ArrayList.
int indexOf(Object o) It is used to return the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element.
void trimToSize() It is used to trim the capacity of this ArrayList instance to be the list’s current size.

 

Java Non-generic Vs Generic Collection

Java collection framework was non-generic before JDK 1.5. Since 1.5, it is generic.

Java new generic collection allows you to have only one type of object in collection. Now it is type safe so typecasting is not required at run time.

Let’s see the old non-generic example of creating java collection.

ArrayList al=new ArrayList();//creating old non-generic arraylist

Let’s see the new generic example of creating java collection.

ArrayList<String> al=new ArrayList<String>();//creating new generic arraylist

In generic collection, we specify the type in angular braces. Now ArrayList is forced to have only specified type of objects in it. If you try to add another type of object, it gives compile time error.

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Java ArrayList Example


package collections.arraylistdemo;


import java.util.*;

public class TestCollection1 {
    public static void main(String args[]) {
        ArrayList<String> list = new ArrayList<String>();//Creating arraylist
        list.add("Ravi");//Adding object in arraylist
        list.add("Vijay");
        list.add("Ravi");
        list.add("Ajay");
        //Traversing list through Iterator
        Iterator itr = list.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

 

       Ravi
       Vijay
       Ravi
       Ajay

Two ways to iterate the elements of collection in java

There are two ways to traverse collection elements:

  1. By Iterator interface.
  2. By for-each loop.

In the above example, we have seen traversing ArrayList by Iterator. Let’s see the example to traverse ArrayList elements using for-each loop.

Iterating Collection through for-each loop

package collections.arraylistdemo;

import java.util.*;

public class TestCollection2 {
    public static void main(String args[]) {
        ArrayList<String> al = new ArrayList<String>();
        al.add("Ravi");
        al.add("Vijay");
        al.add("Ravi");
        al.add("Ajay");
        for (String obj : al)
            System.out.println(obj);
    }
}

 

       Ravi
       Vijay
       Ravi
       Ajay

User-defined class objects in Java ArrayList

Let’s see an example where we are storing Student class object in array list.

package collections.arraylistdemo;

import java.util.*;

class Student {
    int rollno;
    String name;
    int age;

    Student(int rollno, String name, int age) {
        this.rollno = rollno;
        this.name = name;
        this.age = age;
    }
}

public class TestCollection3 {
    public static void main(String args[]) {
        //Creating user-defined class objects
        Student s1 = new Student(101, "Sonoo", 23);
        Student s2 = new Student(102, "Ravi", 21);
        Student s3 = new Student(103, "Hanumat", 25);
        //creating arraylist
        ArrayList<Student> al = new ArrayList<Student>();
        al.add(s1);//adding Student class object
        al.add(s2);
        al.add(s3);
        //Getting Iterator
        Iterator itr = al.iterator();
        //traversing elements of ArrayList object
        while (itr.hasNext()) {
            Student st = (Student) itr.next();
            System.out.println(st.rollno + " " + st.name + " " + st.age);
        }
    }
}

Test it Now

       101 Sonoo 23
       102 Ravi 21
       103 Hanumat 25

Example of addAll(Collection c) method

package collections.arraylistdemo;

import java.util.*;

public class TestCollection4 {
    public static void main(String args[]) {
        ArrayList<String> al = new ArrayList<String>();
        al.add("Ravi");
        al.add("Vijay");
        al.add("Ajay");
        ArrayList<String> al2 = new ArrayList<String>();
        al2.add("Sonoo");
        al2.add("Hanumat");
        al.addAll(al2);//adding second list in first list
        Iterator itr = al.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

 

       Ravi
       Vijay
       Ajay
       Sonoo
       Hanumat

Example of removeAll() method

package collections.arraylistdemo;

import java.util.*;

public class TestCollection5 {
    public static void main(String args[]) {
        ArrayList<String> al = new ArrayList<String>();
        al.add("Ravi");
        al.add("Vijay");
        al.add("Ajay");
        ArrayList<String> al2 = new ArrayList<String>();
        al2.add("Ravi");
        al2.add("Hanumat");
        al.removeAll(al2);
        System.out.println("iterating the elements after removing the elements of al2...");
        Iterator itr = al.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }

    }
}

 

       iterating the elements after removing the elements of al2...
       Vijay
       Ajay


Example of retainAll() method

package collections.arraylistdemo;

import java.util.*;

public class TestCollection6 {
    public static void main(String args[]) {
        ArrayList<String> al = new ArrayList<String>();
        al.add("Ravi");
        al.add("Vijay");
        al.add("Ajay");
        ArrayList<String> al2 = new ArrayList<String>();
        al2.add("Ravi");
        al2.add("Hanumat");
        al.retainAll(al2);
        System.out.println("iterating the elements after retaining the elements of al2...");
        Iterator itr = al.iterator();
        while (itr.hasNext()) {
            System.out.println(itr.next());
        }
    }
}

 

       iterating the elements after retaining the elements of al2...
       Ravi

Java ArrayList Example: Book

Let’s see an ArrayList example where we are adding books to list and printing all the books.

package collections.arraylistdemo;

import java.util.*;

class Book {
    int id;
    String name, author, publisher;
    int quantity;

    public Book(int id, String name, String author, String publisher, int quantity) {
        this.id = id;
        this.name = name;
        this.author = author;
        this.publisher = publisher;
        this.quantity = quantity;
    }
}

public class ArrayListExample {
    public static void main(String[] args) {
        //Creating list of Books
        List<Book> list = new ArrayList<Book>();
        //Creating Books
        Book b1 = new Book(101, "Let us C", "Yashwant Kanetkar", "BPB", 8);
        Book b2 = new Book(102, "Data Communications & Networking", "Forouzan", "Mc Graw Hill", 4);
        Book b3 = new Book(103, "Operating System", "Galvin", "Wiley", 6);
        //Adding Books to list
        list.add(b1);
        list.add(b2);
        list.add(b3);
        //Traversing list
        for (Book b : list) {
            System.out.println(b.id + " " + b.name + " " + b.author + " " + b.publisher + " " + b.quantity);
        }
    }
}

 

Output:

101 Let us C Yashwant Kanetkar BPB 8
102 Data Communications & Networking Forouzan Mc Graw Hill 4
103 Operating System Galvin Wiley 6

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